[D13]

Do the math.
TH400 to 700R4, with lock up, is about .7 ratio change (since the lockup eliminates the slip of the converter too).
Lets say getting 10mpg. 10/.7=14mpg.
Let's assume its' your DD and sees 15,000 miles per year. 15,000/10 = 1,5000 gallons of gas, at $3.50, that $5250 a year.
15,000/14 = 1070 gallonss, at $3.50, that $3750 a year in gas.
So that's $1500 a year, assuming you can drive it at 60mph and 1540! RPM. Which you can't - you need to be around 1650 rpm at 60mph to tolerate the drive. Really, you need to go to a part time case to get reasonable fuel economy.
Here's the key. Aim to make 1600RPM at 55MPH, and find the ratios that work. Think of the 700R4 as a .72 ratio and play with tire size and gearing.